I've done a little bit of this and Claude is pretty great. Take the app and let Claude run wild with it. It does require you to be relatively familiar with the app as you may need to guide it in the right direction.

I was able to get it to rebuild and hack together a .NET application that we don't have source for. This was done in a Linux VM and it gave me a version that I could build and run on Windows.

We're past the point of legacy blackbox apps being a mystery. Happy to talk more, my e-mail is available on my profile.